1. Field of the Invention
The present invention relates to a magnetic recording member preferably used in magnetic recording medium as, for example, magnetic discs, magnetic drums, magnetic tapes, and to the method for preparation thereof.
2. Description of the Prior Arts
Recently, magnetic tape, floppy disc and hard disc in which a magnetic metal film is used as a recording medium have been rapidly and widely developed as the magnetic recording member having a high recording density. It brings three great advantages to use the magnetic metal film as the recording medium. That is, first of all it is possible to reduce the thickness of the recording layer owing to its large saturated magnetic flux density. In the second place, because a high coercive force can be obtained, it is suitable for high density recording. Thirdly, the magnetic metal film can be easily prepared by means of a method such as plating, sputtering and vapor deposition etc.
The demand for high recording density in the magnetic recording apparatus has gradually increased in these years. In order to obtain such a high recording density, it is necessary to improve the properties of the magnetic recording member and those of the magnetic head, to reduce the thickness of the magnetic recording medium as well as to reduce the distance between the head and the recording member. The magnetic recording member such as magnetic tapes and floppy discs is, in general, used in contact with or quasi contact with a magnetic head to obtain the optimum recording density.
Further, in the hard disc, the magnetic head is used, being slightly levitated from the surface thereof. In order to reduce the distance between the surface of the hard disc and the magnetic head, the load exerted on the magnetic head is raised and also CSS (contact-start-stop) type head levitation system is adopted. Therefore, it sometimes causes friction or wear defects such as head crush on the surface of the magnetic recording layer.
As a solution for such problems, it has been proposed that a protective layer is applied onto the magnetic layer so that the mechanical durability is improved. It is required for such a protective layer to be excellent not only in mechanical durability such as wear resistance, but also in chemical durability (weathering resistance) such as corrosive resistance of the magnetic layer.
To date, there have been proposed various kinds of the protective layer and the method for preparation thereof as follows;
(1) A method wherein a high-hardness metal such as Rh, Cr is deposited onto the magnetic layer by the electroplating. PA0 (2) A method wherein the surface of a magnetic layer is coated with high-hardness metal such as Cr, W or oxides such as SiO.sub.2, Ald.sub.2 O.sub.3 (Japanese Patent Publication No. 185029/1983), carbides or nitrides of a high hardness (U.S. Pat. No. 4,277,540) by the sputtering or the vacuum deposition method, etc. PA0 (3) A method wherein a protective oxide layer is formed by oxidizing the surface of a magnetic layer made of Co or Co-Ni in an oxidizing atmosphere, according to the heat-oxidizing (U.S. Pat. No. 4,124,736), the anodizing or the acid etching method. PA0 (1) The protective metal layer may be formed with a very low thickness and suitable for recording system of high density. Once the head crush occurs due to a contact with the magnetic head, however, fine metallic powder resulting from the head crush adheres to the magnetic head to further develop the crush with the result that it gives sometimes fatal damage to the recording medium. Moreover, due to its insufficient weathering resistance under a high relative moisture condition, the protective metal layer suffers from pit-like corrosion which causes readily the wear damage such as the head crush. PA0 (2) Among the non-metallic protective layer such as oxide, nitride, carbide, etc., the protective layer of SiO.sub.2 has been most widely used, because it is excellent in weathering resistance and wear resistance. Since SiO.sub.2 presents a high absorption activity, however, fluids such as the lubricant, its solvent or the moisture in the air tend to spread between the magnetic head and the protective layer of SiO.sub.2, when the rotation of the recording member is stopped. Thus, the so-called "head stick" phenomenon readily arises since the absorptive strength increases due to the surface tension of the fluid. PA0 (3) In the heat-oxidizing method where the metallic surface of the magnetic layer is oxidized to form a protective oxide film on the surface of the magnetic layer, the heating must be generally conducted up to 300.degree. C., in order to avoid the thermal deformation of the substrate and the degradation of the magnetic layer. At such a relatively low temperature, it is difficult to form a uniform oxide layer with a metal of a high melting point such as Co or Ni. Moreover, since the heat diffusion is insufficient in the treatment of such a low temperature, there remain, even after the treatment, micro defects such as pores and cracks and thermal stress in the magnetic layer. It has been pointed out that, due to the above defects, the magnetic recording member made by the heat-oxidizing method of the prior art does not have a uniform reproductivity and it readily suffers from the local corrosion of surface film-pores type.
The above methods of the prior art present problems as follows:
There is another problem to be solved in the magnetic recording member with the lubricant layer. As explained in the above, the magnetic recording member of the type using magnetic metallic medium is sometimes coated with a lubricant for heightening the resistance to the wear due to the contact with the magnetic head and to the head crush.
For heightening the resistance to the head crush and to the wear, it is effective to planish the surface of the protective layer. When the planished protective layer is coated with the lubricant, however, the solvent and the surface-active agent contained in the lubricant tend to spread on the interface between the magnetic recording member and the magnetic head so that the absorptive strength increases due to the surface tension thereof, resulting in the following problems:
The so-called "head absorption phenomenon" arises. That is, the magnetic head is sticked by the absorptive stress due to the surface tension of the spreaded fluid. The second problem is the so-called "spin-off phenomenon" of the lubricant. That is, the lubricant tends to be dissipated by the centrifugal force due to the high-speed rotation of the magnetic recording member.
In order to solve the above problems, it has been proposed to mechanically process the surface of the magnetic disc to provide a surface roughness or irregularity (Japanese Patent Publication No. 150582 of 1984).
This method comprises planishing a substrate of a magnetic disc to provide a roughness of 5 nm on the surface thereof; and applying an elastically supported lapping film onto the surface of the substrate, while rotating the substrate. In this case, by controlling the size of the grinding grain on the lapping film and the contact pressure between the film and the substrate, the fine irregularity is formed concentrically on the substrate. The surface irregularity of the substrate may be transcribed successively to the magnetic layer, protective layer and so on, so that the final surface of the magnetic disc has also the fine irregularity.
This method of the prior art, however, is not efficient in the productivity, because the substrate is subjected to the lapping process one by one after it is planished to present a finely processed surface having a surface roughness of about 5 nm. Further the surface roughness obtained by lapping process is transcribed to that of the magnetic layer, so that the magnetic layer presents also surface irregularity with the result that the recording and reproducing characteristic and the SN ratio of the final recording member are lowered.
Although the magnetic disc should present a surface roughness higher than 5 nm in order to prevent the absorption phenomenon of the head and the "spin-off" phenomenon of the lubricant, it is required that the surface roughness of the magnetic disc should be restricted to lower than 10% of the levitation height of the magnetic head in order to obtain high and stable recording and reproducing characteristics and a high mechanical rigidity.
On the other hand, for obtaining the high recording density, it has been required recently that the levitation height of the magnetic head be lowered and that the vertical recording system be employed practically. In order to comply with such requirements, a new process for preparation of protective layer has been earnestly required to provide a magnetic recording member which presents a surface roughness for retaining the lubricant and a high wear resistance.